bug 1218576 - Request an immediate batch send when the batch size hits a high water mark. r=gfritzsche

We've now seen an OOM trying to grow the accumulations array during a test
suite. The g*Accumulations arrays do grow unboundedly in size waiting for the
IPC timer to fire, so we should put a size bound as well as a time bound to
trigger the batch send.

The current size bound is 5K elements. This was chosen out of a hat.

MozReview-Commit-ID: 5HeWdbLfN03

toolkit/components/telemetry/TelemetryHistogram.cpp

@@ -225,6 +225,11 @@ const mozilla::Telemetry::ID kRecordingInitiallyDisabledIDs[] = {
 // timeliness and performance (see bug 1218576)
 const uint32_t kBatchTimeoutMs = 2000;
 
+// To stop growing unbounded in memory while waiting for kBatchTimeoutMs to
+// drain the g*Accumulations arrays, request an immediate flush if the arrays
+// manage to reach this high water mark of elements.
+const size_t kAccumulationsArrayHighWaterMark = 5 * 1024;
+
 } // namespace
 
 
@@ -1315,6 +1320,11 @@ internal_RemoteAccumulate(mozilla::Telemetry::ID aId, uint32_t aSample)
   if (!gAccumulations) {
     gAccumulations = new nsTArray<Accumulation>();
   }
+  if (gAccumulations->Length() == kAccumulationsArrayHighWaterMark) {
+    NS_DispatchToMainThread(NS_NewRunnableFunction([]() -> void {
+      TelemetryHistogram::IPCTimerFired(nullptr, nullptr);
+    }));
+  }
   gAccumulations->AppendElement(Accumulation{aId, aSample});
   internal_armIPCTimer();
   return true;
@@ -1330,6 +1340,11 @@ internal_RemoteAccumulate(mozilla::Telemetry::ID aId,
   if (!gKeyedAccumulations) {
     gKeyedAccumulations = new nsTArray<KeyedAccumulation>();
   }
+  if (gKeyedAccumulations->Length() == kAccumulationsArrayHighWaterMark) {
+    NS_DispatchToMainThread(NS_NewRunnableFunction([]() -> void {
+      TelemetryHistogram::IPCTimerFired(nullptr, nullptr);
+    }));
+  }
   gKeyedAccumulations->AppendElement(KeyedAccumulation{aId, aSample, aKey});
   internal_armIPCTimer();
   return true;